1. Technical Field of the Invention
The present invention relates generally to a gear shift control system for an automatic transmission for vehicles, and more particularly to such a control system designed to control an operation of the automatic transmission when it is required to decelerate the vehicle.
2. Background Art
Usually, an internal combustion engine in automotive vehicles is rotated in a selected one of multiple gear ranges in order to drive road wheels of the vehicle at a desired torque. Specifically, the vehicle has an automatic transmission disposed between a drive shaft and the wheels to provide a given torque to the wheels either in a low-speed range or in a high-speed range
Typical automatic transmissions work to automatically change gear ratios as a function of relation of the position of an accelerator operated by the driver of the vehicle and the speed of the vehicle to transmit torque, as outputted by the internal combustion engine, to the wheels. For instance, the automatic transmission is, as demonstrated in FIG. 14, designed to change the gear ratios at the time when the relation (i.e., a locus) between the position of the accelerator and the speed of the vehicle meets one of upshift gear shift lines US or downshift gear shift lines DS prepared one for each of a set of gear ranges (see Japanese Patent First Publication listed below). The automatic transmission, as taught in this publication, is engineered to inhibit an upshift thereof (i.e., fix a selected one of the gear ranges) or establish a downshift by one gear to enhance the efficiency of engine braking when the vehicle is running on an slope.    Japanese Patent First Publication No. 10-238621.
The driver of the vehicle, as demonstrated by a speed reduction locus DT in FIG. 14, may release the accelerator pedal suddenly for some reason while the vehicle is accelerating, and then depress the brake pedal strongly to decelerate the vehicle rapidly. This causes the speed reduction locus DT to intersect with the upshift gear shift lines US several times. The automatic transmission, thus, upshifts several times in a gear shift control mode even though the speed of the vehicle is not very high, which results in an increased possibility that the speed of the internal combustion engine falls in an engine stall range. Additionally, the speed reduction locus DT also intersects with the downshift gear shift lines DS several times to establish the downshift of the automatic transmission. This results in sequential application of sudden engine braking to the vehicle, which will give occupants of the vehicle an uncomfortable ride and also lead to a bad maneuverability of the vehicle.
When the vehicle is decelerating on a slope, the gear shift control system, as taught in the above publication, works to control the operation of the automatic transmission to provide the engine braking to ensure the continuity of speed reduction control. Specifically, the gear shift control system calculates an inclination of the road and, when the inclination exceeds a given threshold, controls an upshift of the automatic transmission. The gear shift control system also calculates a rate of deceleration of the vehicle and, when such a rate exceeds a given threshold, instructs the automatic transmission to downshift to produce engine braking. Usually, the rate of deceleration is derived by calculating a change in speed of the vehicle for a set period of time. The comparison of the rate of deceleration with the threshold is, therefore, made after a lapse of the set period of time following the start of deceleration of the vehicle. This may cause the automatic transmission to upshift or downshift undesirably depending upon the relation between the speed of the vehicle and the position of the accelerator pedal, which may result in a lack of braking force when the vehicle has started decelerating suddenly during acceleration.
It is, therefore, an object to provide a gear shift control system for an automatic transmission which is mounted in a vehicle along with a power source such as an internal combustion engine or an electric motor and designed to inhibit the automatic transmission from shifting gear ratios undesirably frequently, avoid the stall of the power source without sacrificing a comfortable ride and maneuverability of the vehicle.
According to one aspect of the invention, there is provided a gear shift control system for an automatic transmission mounted in a vehicle to transmit torque, as produced by a power source, to a wheel of the vehicle. The gear shift control system comprises: (a) a speed determiner which works to determine a speed of the vehicle; (b) a deceleration detector which detects deceleration of the vehicle when a braking force is being applied to the wheel; (c) a braking force determiner which determines the braking force applied to the wheel when the deceleration detector is detecting the deceleration of the vehicle; (d) a deceleration determiner which determines a deceleration of the vehicle as a function of the speed of the vehicle, as determined by the speed determiner, and the braking force, as determined by the braking force determiner; and (e) a gear shift controller which determines whether the deceleration of the vehicle, as determined by the deceleration determiner, is greater than a given deceleration threshold. When the deceleration of the vehicle is determined to be greater than the given deceleration threshold, the gear shift controller inhibits the automatic transmission from upshifting gears thereof.
In the first preferred mode of the invention, the vehicle is equipped with a brake pedal which is responsive to depression thereof to produce the braking force. The gear shift control system further includes an amount-of-braking effort determiner which works to determine an amount by which the brake pedal is depressed. The deceleration detector detects the deceleration of the vehicle when the braking force is being applied to the wheel in response to the depression of the brake pedal. The braking force determiner calculates the braking force applied to the wheel when the deceleration detector is detecting the deceleration of the vehicle as a function of the speed of the vehicle, as determined by the speed determiner, and the amount by which the brake pedal is depressed, as determined by the amount-of-braking effort determiner.
In the second preferred mode of the invention, the vehicle is equipped with an accelerator pedal which is responsive to depression thereof to accelerate the vehicle. The gear shift control system may further include an amount-of-acceleration determiner which works to determine an amount by which the accelerator pedal is depressed. The deceleration detector may detect the deceleration of the vehicle when the braking force is being applied to the wheel in response to release of the accelerator pedal. The braking force determiner may calculate the braking force applied to the wheel which arises from the release of the accelerator pedal.
In the third preferred mode of the invention, the vehicle may be equipped with either or both an internal combustion engine and an electric generator which serve to produce the braking force applied to the wheel. The braking force determiner may determine one or the sum of an engine braking force, as produced by the internal combustion engine, and a regenerative braking force, as produced by the electric generator, as the braking force applied to the wheel.
In the fourth preferred mode of the invention, the braking force determiner may determine a regenerative electric energy which is produced by the electric generator and being charged in a storage battery mounted in the vehicle and calculate the regenerative braking force based on the determined regenerative electric energy to determine the braking force.
In the fifth preferred mode of the invention, the given deceleration threshold may be set to change to have one of a plurality of values as a function of the speed of the vehicle. The plurality of values are different from each other. When the deceleration of the vehicle is determined to be greater than a greatest one of the values, the gear shift controller may inhibit the automatic transmission from upshifting gears thereof. When the deceleration of the vehicle is determined to be greater than one of the values other than the greatest one, the gear shift controller permits the automatic transmission from upshifting in a range of a given number of gear ratios. The smaller the values, the greater the given number of gear ratios.